It should be observed that the sweep of an aerodynamic surface serves to specify the orientation of said surface relative to its direction of forward advance.
Thus, a forwardly-swept segment of a blade is directed in the advance direction of the blade, whereas a rearwardly-swept segment is directed in the direction opposite to the advance direction.
It can readily be understood that a radial segment, said for convenience to have zero sweep, is perpendicular to the forward direction of the blade. Such a radial segment of a blade of a main rotor of a helicopter is thus directed along the reference radius of the rotor, hence the use of the term “radial”.
Below, the term “radial segment” is used to specify a segment having zero sweep, and the term “swept segment” is used to designate a segment that is forwardly- and/or rearwardly-swept.
Document FR 2 865 189 discloses a rotorcraft blade that possesses in succession, starting from the blade root: a segment that is radial, and thus of zero sweep; a forwardly-swept segment; and then a rearwardly-swept segment.
A blade of that type is particularly advantageous insofar as its shape significantly reduces the acoustic signature of the rotor fitted therewith, while nevertheless preserving its dynamic and aerodynamic characteristics.
Nevertheless, to ensure that the blade possesses sufficient strength, it is appropriate in particular firstly to take up the centrifugal forces exerted on the blade, and secondly to ensure that the leading edge of the blade can withstand impacts.
Conventionally, a blade comprises:                at least one rigid spar based mainly on unidirectional inorganic fibers or on metal, which spar serves to withstand the centrifugal forces and to transmit them to the rotorcraft hub;        bottom and top covering skins that provide stiffness in flapping and in drag, and that also provide the blade with the ability to withstand twisting, e.g. being made of bidirectional fabric based on inorganic fibers; and        at least one filler box for guaranteeing geometrical continuity of the assembly and stability of the bottom and top covering skins.        
A first type of blade fitted with a spar is known in which the spar extends from the blade root, and in particular from the fastenings between the blade and the hub, along the entire span of the blade, being arranged at the leading edge of the blade. For convenience in the text below, that type of spar is referred to as a “leading-edge” spar.
A leading-edge spar is highly effective in general, and furthermore it protects the leading edge against impacts, but it does not appear to be capable of satisfying the requirements of a blade comprising both a radial segment and a forwardly- and/or rearwardly-swept segment.
Under the effect of centrifugal force, such a blade tends to “unfold” in order to become completely straight. Under such conditions, the leading-edge spar is found to be insufficient, since it too is likely to unfold insofar as, given its location in the leading edge, it also presents a broken-line profile corresponding to the radial and the forwardly- and/or rearwardly-swept segments.
A second type of blade provided with a spar is known in which the spar extends from the blade root along the span of the blade, being arranged inside the blade. For convenience, that type of spar is referred to in the text below as a “central spar”.
For example, a central spar is to be found in document U.S. Pat. No. 5,127,802 which describes a tubular central spar made of glass fibers, which spar is additionally reinforced by top and bottom plates made of carbon fibers.
Since it does not extend along the leading edge of the blade, such a central spar can be radial, i.e. it can be directed along the radius of the rotor.
Nevertheless, if that spar were to be used in a blade presenting a radial segment and a swept segment, the central position of the spar relative to the chord of the blade would vary along its span. For example, if it lies in the middle of the chord in the radial segment it will necessarily come closer to the trailing edge of the blade in a forwardly-swept segment. In contrast, it will necessarily come closer to the leading edge in a rearwardly-swept segment.
Consequently, it is appropriate to overdimension the spar in the forwardly-swept segment, thereby leading to problems of weight and also of centering for the blade, since a blade needs to be centered approximately in the vicinity of its pitch axis.
A third type of blade is known that combines both a leading-edge spar and a central spar along the span of the blade. The leading-edge spar is separated from the central spar by a torsion box having its walls constituted by said spars and the bottom and top covering skins, as disclosed for example in document U.S. Pat. No. 5,346,367.
That solution can be attractive for a blade provided with a radial segment and a swept segment, but it can lead to harmful overdimensioning and to excess weight.
Finally, a fourth type of blade is known that is provided with a spar referred to as a “distributed spar”. Such a spar consists in a leading-edge spar that is extended a little by portions extending against the bottom and top skins of the blade.
More precisely, the distributed spar comprises first inorganic fibers placed flat against the front portion of the top skin and second inorganic fibers placed flat against the front portion of the bottom skin. These fibers meet at the leading edge of the blade to constitute a conventional leading-edge spar.
The distributed spar satisfies present requirements while avoiding a large increase in the weight of the blade.
Nevertheless, there remains the problem of the broken-line shape of the leading edge of a blade that has a radial segment and a swept segment. Having a radial segment followed by at least one forwardly- and/or rearwardly-swept segment leads to the fibers being distributed in extremely complex manner along the span. This makes fabrication difficult to reproduce, and above all difficult to automate, given the degree of precision required in making the blade.